It is desirable to increase the temperature at which an internal combustion engine operates because higher temperatures generally result in more efficient fuel consumption, reduced emissions, and the possibility of waterless uncooled operation. High engine temperatures are generally obtained by insulating the combustion chamber surfaces of the engine. However, if the operating temperature of an engine is increased to about 177.degree. C. (350.degree. F.), conventional organic liquid lubricants begin to burn and break down, forming sludge deposits on the cylinder wall and ring grooves. These deposits increase wear on the cylinder, piston and piston rings, and cause liner scuffing and stuck rings. Polyolester base liquid lubricants can withstand temperatures of up to 316.degree. C. (600.degree. F.), but it is desirable to obtain even higher operating temperatures.
One method of obtaining a high engine operating temperature while minimizing the breakdown of liquid lubricant takes advantage of the fact that the temperature at the top of a piston is always higher than at the bottom of the piston, as shown by FIG. 4. This is because the top of the piston is directly heated by combustion in the cylinder, while the bottom portion of the piston is somewhat "insulated" by the intermediate portion of the piston, and cooled by oil splashed on the bottom of the piston. Along the axial length of the cylinder liner, the temperature also drops rapidly from the TRR point as shown in FIG. 3. Thus, one solution is to eliminate the piston rings ordinarily placed around the upper periphery of the piston. Such a piston is shown in FIG. 4. For this solution to work, the piston rings placed around the lower periphery (lower ring pack) of the piston must include an oil control ring, so that oil does not reach the upper periphery of the piston. This technique prevents the liquid lubricant from reaching the hotter, upper periphery of the piston. Because no lubricant is present in the upper periphery of the piston, it is possible to operate the engine at higher temperatures. In theory, the temperature of the piston rings at their top ring reversal (TRR) point (in the lowered piston pack) will be sufficiently low to prevent breakdown of the liquid lubricant. However, merely eliminating the upper piston rings creates another problem, namely, the creation of a larger annular air gap around the upper circumference of the piston, between the piston and the cylinder. This dead air space results in poor air utilization with resultant poor emission and brake specific fuel consumption.
Another method of achieving higher operating temperatures is to completely eliminate liquid lubricants, and to use solid lubricants instead. Solid lubricants can withstand the higher temperatures, and may be placed on piston ring and cylinder wall surfaces. Examples of such solid lubricants include densified chrome oxide, STELLITE 1 and STELLITE 6 (from Stoody Deloro Stellite, Inc. of Goshen, Ind.), Hastelloy X steel (from Inco Alloys International of Huntington, W. Va.), triboly steel (from Metco, Inc., of Long Island, N.Y.), molybdenum, copper alloyed with lead oxide or titanium, and lithium fluoride alloyed with copper or molybdenum. U.S. Pat. No. 3,675,738 discloses a piston having a solid lubricant disc made of calcium fluoride which engages the cylinder wall. U.S. Pat. No. 3,890,950 discloses a piston having a cylinder engaging surface made of graphite. One advantage of dry lubricants is that particulate emissions resulting from oil consumption are eliminated.
The shortcoming of such solid lubricants is that they have a high (0.20-0.50) coefficient of friction. This is far higher than the coefficient of friction for liquid lubricants, which is about 0.04. Another shortcoming is the high wear rate of solid lubricant alone. Thus, it is desirable to obtain a piston system which simultaneously provides the low friction and long life of liquid lubricants and high temperature capabilities of solid lubricants.
It is also known in liquid lubricated engines to introduce solid lubricants into the combustion chamber to further reduce friction. U.S. Pat. No. 3,994,697 discloses a pellet consisting of a metal and metal salt, such as molybdenum disulfide, which will dissolve when placed in a gas tank and will introduce the metal salt solid lubricant to the combustion chamber through the fuel system. This is believed to reduce friction by the solid lubricant filling and smoothing surface irregularities of metallic components. It is also known to combine liquid and solid lubricants into a single composition, as disclosed in U.S. Pat. Nos. 4,127,491, 4,284,518 and 4,349,444. Although the simultaneous use of liquid and solid lubricants may reduce friction in some instances, they do not permit an increase in engine operating temperature above the limits mentioned above.
More recently, H. E. Sliney of NASA has developed a solid lubricant material (PS212) which was tested in a stirling engine. The "PS200 lubricant comprises Ag, CaF.sub.2 and BaF.sub.2 in a matrix of Cr.sub.3 C.sub.2 rubbing against STELLITE 6B. (Journal of Vacuum Science and Technology, Vol. 4, No. 6, Nov./Dec. 1986), incorporated herein by reference.